1. Field of the Invention
The present invention relates generally to geophysical exploration and more particularly to methods for accurately estimating fluid and overburden pressures in the earth's subsurface on local, prospect and basin-wide scales.
2. Background of the Art
During drilling of a borehole, drilling fluids, usually referred to as “mud,” are circulated in the borehole to cool and lubricate the drill bit, flush cuttings from the bottom of the hole, carry cuttings to the surface, and balance formation pressures encountered by the borehole. It is desirable to keep rotary drilling mud weights as light as possible, but above the formation pore fluid pressure, to most economically penetrate the earth; heavier muds may break down rocks penetrated by the borehole and thereby cause loss of mud. Mud weight is carefully monitored and may be increased during drilling operations to compensate for expected increases in the formation fluid pressure. In some areas, however, there may be unexpected abnormal increases in pressure, with depth such that mud weight does not compensate pressure; the result can be blowout of the well.
Normal pressures refer to formation pressures that are approximately equal to the hydrostatic head of a column of water of equal depth. If the formation were opened to the atmosphere, a column of water from the ground surface to the subsurface formation depth could balance the formation pressure. In many sedimentary basins, shallow predominantly sandy formations contain fluids that are under hydrostatic pressure.
At a number of offshore locations, abnormally high pore pressures have been found even at relatively shallow sub-sea bottom depths (less than about 1500 meters). This could occur if a sand body containing large amounts of water is covered by silt or clay and buried. The dewatering of clays may result in the formation of relatively impermeable shale layers that slow down the expulsion of water from the underlying sand. The result of this is that the sand may retain high amounts of fluid and the pore pressure in the sand exceeds that which would normally be expected from hydrostatic considerations alone, i.e., the fluid pressure exceeds that which would be expected for a column of water of equivalent height. This phenomenon of overpressuring is well known to those versed in the art and is commonly referred to as “geopressure.”
It is desirable to set casing in a borehole immediately above the top of a geopressured formation and then to increase mud weight for pressure control during further drilling. Setting a casing string which spans normal or low pressure formations permits the use of very heavy drilling muds without risking breaking down of borehole walls and subsequent lost mud in the shallower interval. On the other hand, should substitution of heavy drilling mud be delayed until the drill bit has penetrated a permeable overpressurized formation (e.g., sandstone), loss of well control or blowout may occur.
In areas where there is reason to suspect existence of such high pressure formations, various techniques have been followed in attempts to locate such geopressure zones. For example, acoustic or electric logs have been run repeatedly after short intervals of borehole have been drilled or are acquired using measurement-while-drilling techniques, and a plot of acoustic velocity or electrical resistance or conductivity as a function of depth has been made. Abnormal variations of acoustic velocity and/or electrical properties obtained by logging may indicate that the borehole has penetrated a zone of increasing formation pressure. Such techniques are very expensive and time-consuming and cannot predict what pressures will be encountered ahead of the bit.
Several methods are known in the art for estimating pore pressures in formations, using well log data and also from seismic survey information. One such method is well known in the art as the “Eaton” method, and is described at Eaton, “The Equation for Geopressure Prediction from Well Logs” SPE 5544 (Society of Petroleum Engineers of AIME, 1975). The Eaton method of determining pore pressures begins with determination of the so-called “normal compaction trend line” based upon sonic, resistivity, conductivity, or d-exponent data obtained by way of well logs. The normal compaction trend line corresponds to the increase in formation density (indicated by sonic travel time, resistivity or conductivity) that would be expected as a function of increasing depth due to the increasing hydrostatic pressure that forces fluids out from the formations and thus decreases the sonic travel time (increases the velocity), assuming the absence of geopressure. This normal compaction trend line may be determined solely from the sonic travel time, conductivity, or resistivity well log data, or may be adjusted to reflect extrinsic knowledge about the particular formations of interest. The Eaton method calculates pore pressure by correlating the measured density information, expressed as an overburden gradient over depth, to deviations in measured sonic travel time, (or electrical resistivity or conductivity) from the normal compaction trend line at specific depths. The pore pressure calculated from the Eaton equations has been determined to be quite accurate, and is widely used in conventional well planning.
Specifically, the Eaton method determines a pressure gradient according to the relation                               G          p                =                              G            0                    -                                                    (                                                      G                    0                                    -                                      G                    n                                                  )                            [                                                Δ                  ⁢                                                                           ⁢                                      t                    normal                                                                    Δ                  ⁢                                                                           ⁢                                      t                    observed                                                              ]                        3                                              (        1        )            where Gp is the formation pressure gradient (psi/ft), Go is the overburden gradient, GN is the normal gradient, Δtnormal is the normal transit time and Δtobserved is the observed transit time.
However, application of the Eaton method has been limited to the immediate locations of existing wells, as it depends on well log data. It is of course desirable to estimate pore pressure at locations at the sites of proposed new wells, and thus away from currently existing wells, particularly to identify locations at which production will be acceptable at a low drilling cost (e.g., minimal use of intermediate casing). In addition, knowledge of pore pressure at locations away from existing wells enables intelligent deviated or offset drilling, for example to avoid overpressurized zones.
Kan (U.S. Pat. No. 5,130,949) teaches a method in which seismic data is combined with well log data to generate a two- dimensional geopressure prediction display; this permits deviated and horizontal well planning plus lithology detection. Shale fraction analysis, compaction trend, and seismic velocity may be automatically or interactively generated on a computer work station with graphics displays to avoid anomalous results. Corrections to velocity predictions by check shots or VSP, and translation of trend curves for laterally offset areas increases accuracy of the geopressure predictions. In particular, Kan '949 determines the transit time from sonic logs for compressional waves in predominantly shaly sections and expresses the pore pressure gradient (PPG) in terms of the transit time departure from the compaction trend line, δΔt, asPPG=0.465 +C1(δΔt)+C2(δΔt)2  (2)
Coefficient C1 typically varies from 0.002 to 0.02 if the transit time is expressed in microseconds per foot and the PPG is measured in psi/ft. C2 may be positive or negative.
Kan '949 also teaches the use of vertical seismic profile (VSP) data for calibration of the sonic log data. Kan (U.S. Pat. No. 5,343,440) and Weakley (U.S. Pat. No. 5,128,866) further teach the use of coherency analysis of surface seismic data for determination of interval velocities.
Scott (U.S. Pat. No. 5,081,612) teaches a variation of the Eaton method in which an equation of the formVc=V1(1−a1L−a2φ+a3P)  (3)where a1, a2 and a3 are constants, Vl is a constant, Vc is calculated velocity, L is the lithology of the formation, φ is the porosity and P is the effective pressure (difference between the overburden pressure and the formation fluid pressure). The compaction of the sediments is governed by an equation of the formφ=φ0e−a4P  (4)
A reference model for the sedimentary basin is developed assuming compaction under hydrostatic pore pressure. A reference effective pressure and a reference velocity profile are obtained. An iterative procedure is used in which the lithology may be varied with depth and the reference velocity profile is compared to a velocity profile obtained from seismic data.
In addition to undercompaction, Bowers (U.S. Pat. No. 5,200,929) discusses a second cause of overpressuring. Abnormally high pressure can also be generated by thermal expansion of the pore fluid (“aquathermal pressuring”), hydrocarbon maturation, charging from other zones, and expulsion/expansion of intergranular water during clay diagenesis. With these mechanisms, overpressure results from the rock matrix constraining expansion of the pore fluid. Unlike undercompaction, fluid expansion can cause the pore fluid pressure to increase at a faster rate than the overburden stress. When this occurs, the effective stress decreases as burial continues. The formation is said to be “unloading.” Since sonic velocity is a function of the effective stress, the velocity also decreases and a “velocity reversal zone” develops. A velocity reversal zone is an interval on a graph depicting sonic velocity as a function of depth along a well in which the velocities are all less than the value at some shallower depth.
A large portion of the porosity loss that occurs during compaction is permanent; it remains “locked in” even when the effective stress is reduced by fluid expansion. A formation that has experienced a greater effective stress than its current value will be more compacted and have a higher velocity than a formation that has not. Consequently, the relationship between sonic velocity and effective stress is no longer unique when unloading occurs. In other words, for every effective stress, there is no longer one unique sonic velocity. The sonic velocity follows a different, faster velocity-effective stress relationship during unloading than it did when the effective stress was building. This lack of uniqueness is called “hysteresis.” Since fluid expansion causes unloading, while undercompaction does not, hysteresis effects make the sonic velocity less responsive to overpressure generated by fluid expansion. As a result, the pore fluid pressure corresponding to a given sonic velocity at given depth within a velocity reversal zone can be significantly greater if the overpressure was caused by fluid expansion rather than undercompaction. Therefore, the sonic velocity of an overpressured formation is indirectly dependent upon both the magnitude and the cause of overpressure. To account for different causes of overpressuring, Bowers teaches the use of two different velocity-effective stress relations: one relation applies when the current effective stress is the highest ever experienced by a subterranean formation and a second relation that accounts for hysteresis effects is used when the effective stress has been reduced. Pore fluid pressure is found by subtracting the computed effective stress from the overburden stress. Bowers uses a relationship of the formV=C+A[σmax(σ/σmax)(1/U)](1/B)  (5)for the effect of unloading. In eq. (5)σ,max is the maximum stress to which the rock has been subjected. The unloading curve parameter U is a measure of how plastic the sediment is, with U=1 and U=∞ defining the two limiting cases. For U=1, the unloading curve is the same as the loading curve whereas for U=∞> the velocity remains fixed at a value Vmax determined by the stress σ,max.